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http://wstiac.alionscience.com The WSTIAC Quarterly,Volume 9,Number 145INTRODUCTIONThe military is moving toward more electrical platforms.To effec-tively sustain US military superiority the Department of Defensecontinues to utilize the latest advances in state-of-the-art equip-ment.Invariably,these advanced systems continue to require anincrease in energy and power density while maintaining safety,reli-ability,size and weight.Military platforms such as warships,tanks,and airplanes,continue to require higher power to enable electri-cal powered weapons and detection systems for both defensive andoffensive missions.The need for more powerful detection systems,communication systems,and more demanding auxiliaries alsocontributes to the demand for reliable,efficient,and clean powerand energy.The Defense Advanced Research Projects Agency (DARPA) iscurrently funding the Wide Band Gap High Power ElectronicsProgram and the Integrated High Energy Density CapacitorProgram.The success of these programs depends upon the abili-ty to integrate new materials into high power electrical systemcomponents.Power electronics* and capacitors are two of themajor components that make up allsolid state power distribution sys-tems.The objectives of DARPA’sprograms in these areas are toincrease power and energy densitythrough materials,processing,andpackaging innovations.For high-powered,hydrocarbon-fueled plat-forms,these programs drive thedevelopment of materials that havehigher efficiencies and performancecapabilities for power electronics andpassive devices.This article providesan overview of some of the efforts to enhance military high powerelectronics and capacitors through new and improved materials.SEMICONDUCTOR MATERIALS FORMILITARY HIGH POWER ELECTRONICS SYSTEMSSolid state power electronics provides enhanced design flexibilityand greater control of electrical power than analog systems.Increasingly,solid state silicon-based semiconductors are no longerable to meet the increased power demands of military platforms.Specifically,the need for higher voltages drives the complexity ofsilicon-based systems.A new class of semiconductor devices,basedon silicon carbide (SiC),is now emerging into the market to meetthe demands of the future military’s high power converters,directcurrent (DC) distribution systems,electromagnetic guns,highenergy lasers and propulsion systems.Intrinsic Properties of SiCSemiconductor materials are based on covalent bonds whereby theelectrons in the outer shell are shared between host atoms.Elements in the upper rows on the Periodic Table have smalleratomic radii and stronger interatomic bond strength compared tothose elements located in rows below these elements.The strongercovalent silicon-carbon bond in SiC results in a higher energybandgap in the SiC semiconductor material,hence the name widebandgap material.This bandgap is a fundamental characteristic ofsemiconductor materials because it is the energy needed to excitean electron from the conduction band into the conductive band.The three times higher band gap of SiC (3.28 electron-volts (eV)for 4H-SiC) compared to silicon (1.12 eV) results in a breakdownelectric field in SiCthat is ten times higher than that of silicon.Thisdramatically higher breakdown field in SiC,in turn,makes it pos-sible to reduce the thickness of the drift region of a SiC powerdevice by a factor of ten,resulting in a significantly reduced transittime for the carriers across the drift region of the device.This ulti-mately results in much faster switching and lower on-resistance forSiC power devices.This higher breakdownfield,coupledwith the higher current densities thatcan be achieved in SiC power devicesdue to the higher thermal conductivityof SiC,means that it is feasible toreplace silicon bipolar devices (e.g.,Si insulated gate bipolar transistors(IGBTs) and PiN diodes) with SiCunipolar devices (e.g.,SiC depletionmode metal-oxide semiconductorfield-effect transistors (DMOSFETs)and Schottky diodes) in high voltagepower electronics systems resulting in lower weight and volume asshown in Figure 1.SiC power devices have the added advantage ofbeing capable of high temperature operation up to 225°C comparedwith the 125°Coperating limit of silicon power devices.This not onlysignificantly reduces the cooling requirements for SiC power devices,but also enhances their survivability in the event loss of cooling.Material Development StatusSignificant advances in the quality of SiC substrates and epitaxiallayers have been made over the last decade.The catastrophicmicro-pipe defects shown in Figure 2 have been reduced to anaverage of <0.7/cm2for 100 mm 4HN-SiC wafers as shown inFigure 3.There remains a need to reduce 1c screw dislocations toless than 100/cm2.At high voltage levels (10 kV) 1c screw dislo-cations cause unacceptable leakage current as shown in Figure 4.Sharon Beermann-CurtinDefense Advanced Research Projects AgencyArlington,VAFigure 1.Comparison of size of silicon and silicon carbideconverters courtesy of GE-GRC.http://ammtiac.alionscience.com/quarterlyhttp://ammtiac.alionscience.com/quarterlyThe AMMTIAC Quarterly,Volume 4,Number 1 http://ammtiac.alionscience.com46Basal Plane Defects are also present in SiC substrates but arehandled through processing techniques to bend themto the outeredges of the boule†.DIELECTRICS FOR CAPACITORSTo meet the high power demands for the future,improved passiveelectrical components are needed to keep pace with technicalimprovements in the state-of-the-art active power electronics.Today’s capacitors take up to 50% of the volume of high powerelectrical systems and are a driving factor in thermal managementoverhead.Capacitor research today is attempting to provide energydense capacitors beyond the capability of 1-2 J/cm3packaged toenergy densities of up to 20 J/cm3packaged with high temperaturecapabilities (200°C),low losses (0.1%dielectric loss),and the abil-ity for manufacturing scalability.A capacitor’s performance isdependent upon the dielectric materials incorporated.To reach the20 J/cm3packaged goal new dielectric materials will need to bedeveloped in either polymer or ceramic materials with newprocess-es and innovative configurations for higher packing density.Progress is being made towards a class of high power,high temper-ature capacitors that will enable future electronic weapons andpulsed power systems as well as more conventional high power dis-tribution systems into smaller weight and volume.Intrinsic Material PropertiesThe electrical energy stored in the electric field between the plateof an ideal capacitors (Figure 5) is in large part determined by twomaterial parameters,permittivity and breakdown field strength,and can be given by equation 1.(1)WhereU - energy density (J/cm3),ε - relative material permittivityεo- permittivity of free space (8.85418782 × 10-12m-3kg-1s4A2)Emax(V/µm) - maximumfield strength before material breakdownPermittivity can be described asthe ability of the material to polar-ize in response to an electric fieldthrough separation of ions,twist-ing permanent dipoles in the formof chemicals bonds,and perturb-ing electron orbitals.Greaterpolarizability results in higherpermittivity.Dielectric breakdownstrength can be described as theamount of electric field a materialcan handle before the electric fieldfrees bound electrons.These elec-trons become accelerated and freeother electrons through the mate-rial causing failure.Material Development StatusCurrently,there are research initia-tives to achieve high temperature,high energy density with longlifetime,fast discharge rate,high voltages,and low loss capacitorobjectives through structural configurations of multiple materi-als.One example is the use of polymer extrusion technology tofabricate nanolayer structures of alternating polymer with differ-ent electrical properties.One polymer is chosen with high per-mittivity and the other possesses high breakdown strength.Theresultant composite is an effective media with an overallincreased energy density through the combined materials.Additionally,the multi-layered structure provides a barrier to thepropagation of an electrical breakdown.Challenges include theability to lay thin layers in a uniformmanner over large areas andextrusion of high temperature polymers.Another approachtoward high energy dense capacitor dielectrics uses a compositesystem of both polymer and ceramic dielectrics in an attempt totake the best properties of each and achieve a higher energy den-Figure 4.Reduction of leakage current when low1c dislocation(<200/cm2) processes are used.(Courtesy of CREE)Figure 5.Schematic of anideal parallel plate capacitor.Charge separation within theparallel-plate causes an inter-nal electric field.A dielectricinside reduces the field andincreases the capacitance.Figure 2.Micro pipe defects.(Courtesy of CREE)Figure 3.Micro pipe defects in 100 mmSiC wafers.Charge+QElectricfield EPlatearea APlate separation d100 mm3-inchLeakage Current (log scale) at 10 kVStandardLow 1c100nA-215nA 215nA-464nA464nA-1µA1µA-2.15µA2.15µA-4.64µA4.64µA-10µA10µA-21.5µA21.5µA-46.4µA46.4µA-100µA100µA-215µA215µA-464µA4540353025201510501µA10µA100µADiodeCount2001 2002 2003 2004 2005 2006 2007 2008Year200100502010521MedianMicropipeDensity,cm2-Q100 mm work supported by ARL MTO (W911NF-04-2-0021) and DARPA (N00014-02-C-0306)U =_______2εεnE2maxhttp://ammtiac.alionscience.com/quarterlyhttp://ammtiac.alionscience.com/quarterlysity.The polymer host provides the high breakdown strengthwhile ceramic nanoparticles embedded within the polymer lendthe high permittivity.There is also research using anti-ferrolec-tric nanoparticles to improve the energy density of dielectricmaterials (see Figure 5).As the electric field is increased,a phasechange occurs within the material to enhance the permittivity ina nonlinear manner due to the polarization of the unalignedanti-ferroelectrics figure.The size of the anti-ferroelectricnanoparticle can be tailored to create an enhanced permittivitywhen high electric fields are applied.The challenges for embed-ding particles into polymers includes homogeneous dispersion aswell as optimization of loading.Lastly,research is currently underway to improve the energydensity and reliability of ceramic capacitors.Ceramics inherent-ly possess a high permittivity and high temperature capability.Current progress is focused on improving the breakdownstrength and lowering the losses.It has been shown that ceramicmaterial sintering parameters can be controlled to produce nano-grain ceramics.The ceramic grains on the order of 300 nmindeed provide increased breakdown strength and longeroperating lifetime.Challenges for this system include controlof material defects and impurities.CONCLUSIONSAs silicon carbide reaches maturity,both in materials processingand in device manufacturing,it will become prolific in commercialand military high power applications.The advances in materialprocessing have reduced the defects such that the higher yield hasreduced cost and made it an attractive alternative for lowpower cir-cuits in which power efficiency is highly valued (e.g.,commerciallaptops).Recent advances in the development and testing of highpower modules are realizing the reduced size and weight that sili-con carbide brings to the table.In the future,power applicationsthat require efficiency and clean power more than 10 kWwill rou-tinely incorporate silicon carbide switches over silicon.Magneticmaterial improvements will also have the effect of allowing smaller,more capable systems in the future to meet the ever growing needfor higher and more efficient power.The ability to integrate theactive and passive electrical components into smarter,more modu-lar circuits will change the way electrical systems are designed.NOTES & REFERENCES* Power electronics involves the conversion and control of electrical power.† Boule is a single crystal formed synthetically.European Conference on Silicon Carbide and Related Materials (2008ECSCRM) in Barcelona,Spain (September,2008).Berkman,E.,R.T.Leonard,M.J.Paisley,et al.,Defect Control in SiCManufacturing,Cree,Inc.,USA.Wolak,M.A.,M-J.Pan,A.Wan,J.S.Shirk,M.Mackey,A.Hiltner,E.Baer,and L.Flandin,Dielectric Response of Structured MultilayeredPolymer Films Fabricated by Forced Assembly,Applied Physics Letters,Vol.92,113301,2008.Huang C.,Q.M.Zhanga,J.Y.Li,and M.Rabeony,Colossal Dielectricand Electromechanical Responses in Self-Assembled PolymericNanocomposites,Applied Physics Letters,Vol.87,182901,2005.Tan,D.Q.and F.Dogan,DSOPower Review Open Session,San Antonio,TX,January 26-30,2009.http://wstiac.alionscience.com The WSTIAC Quarterly,Volume 9,Number 147Ms.Sharon Beermann-Curtin is currently a Program Manager in the Defense Systems Technology Office (DSO) at the Defense AdvancedResearch Projects Agency (DARPA).Her portfolio of programs is focused on power and energy,including alternative energies,batteries,fuelcells,thermoelectrics,magnetics,high power capacitors and high power semiconductor efforts.Prior to DARPA she spent 10 years at theOffice of Naval Research (ONR) where she was most recently the Acting Deputy Department Head of the Materials and Physicals Sciences,and Ship Hull Mechanical & Electrical Science & Technology Department.She was a visiting scholar in the massachusetts Institute ofTechnology (MIT) Ocean Engineering Department (13-A program) in 2002.From 1999-2001 she was the Chief Technology Officer for theProgram Executive Office–Aircraft Carriers responsible for the transition of new technologies to both in-service and future aircraft carriers.Earlier positions held at ONR include Technology Manager for Ship Systems in the Hull,Mechanical and Electrical S&T Division,andUnderwater Weapons Countermeasures Program Manager.Ms.Beermann-Curtin holds a master’s and bachelor’s degree in ElectricalEngineering.Figure 5.Schematic depicting the increased energy density fromferroelectric to anti-ferroelectric behavior.FerroelectricAnti-ferroelectricElectricEnergyW=qQQVVdq0ElectricEnergyVq∫W=QVdq∫0Qhttp://ammtiac.alionscience.com/quarterlyhttp://ammtiac.alionscience.com/quarterly